This invention relates to apparatuses and a method for examining magnetic properties of objects, in particular sheet material, such as bank notes. The apparatuses comprise a magneto-optic layer whose optical properties are influenceable by the magnetic properties of the object, a light source for producing light that is coupled into the magneto-optic layer, and a detector for detecting light that is transmitted and/or reflected by the magneto-optic layer.
To guarantee high forgery-proofness, bank notes are provided with magnetic features, among other things. During automated bank note testing in bank note processing machines, bank notes are therefore also examined for their magnetic properties to distinguish counterfeits or suspected counterfeits from authentic bank notes.
Examination of the magnetic properties of bank notes is usually effected in this connection using inductive measuring heads, Hall elements or magnetoresistive elements, such as field plates or thin permalloy layers.
In addition, it is known to examine the magnetic properties of bank notes using magneto-optic layers. A suitable apparatus is known for example from German laid-open print DE 197 18 122 A1. A magneto-optic reflector layer with a high magnetic Kerr effect is illuminated with polarized light and the reflected light detected after passing through a polarizing filter. If a bank note under examination is brought close behind the reflector layer, the magnetic leakage flux of the magnetic areas of the bank note influences the optical behavior of the reflector layer, thereby changing the polarization direction of the detected light. From the measured change of polarization one can then infer the magnetic properties of the sheet material.
Compared to other measuring methods and assemblies, e.g. inductive measuring heads, the use of magneto-optic layers has the advantage that they allow high spatial resolution and the measurement of magnetic flux is independent of the speed of the bank note relative to the measuring system.
Examination of the magnetic properties of bank notes by machine involves in particular the problem that very small magnetic flux densities must be detected to be able to guarantee a sufficiently precise and reliable check of authenticity. This is because, firstly, the leakage flux caused by the individual magnetic areas of the bank notes is very small and, secondly, the typical distances between bank note and magneto-optic layer cannot be reduced at will due to the high transport speed required in bank note processing machines, as this would otherwise lead to elevated wear of the bank notes to be checked and individual sensor components and in addition result in an elevated risk of jams.
It is the problem of the present invention to provide apparatuses and a method that allow more precise and reliable examination of magnetic properties of sheet material.
This problem is solved by the apparatus according to claims 1 or 9 and by the method according to claim 9.
The solutions to the problem by the apparatuses according to claims 1 and 9 are based on the common inventive idea of increasing the change, i.e. rotation, of the polarization direction of the light coupled into the magneto-optic layer. This increases the sensitivity of the measuring apparatus so that even very small magnetic fields can be examined with sufficiently high precision and reliability. The rotation of the polarization direction is increased in the apparatus according to claim 1 by increasing the optical path length of the light passing through the magneto-optic layer. In the apparatus according to claim 9 this is attained by a suitable structure of the system consisting of magneto-optic layer and substrate.
According to claim 1, it is provided that the light source and the magneto-optic layer are so disposed that the direction of propagation of the light coupled into the layer extends substantially parallel to a base surface of the magneto-optic layer. Light beams also extend parallel to the base surface of the layer according to the invention when they are inclined relative to the base surface of the layer slightly, i.e. up to an angle of a few degrees.
As is well-known, the angle of polarization rotation is proportional to the optical path length of the light passing through the magneto-optic material in the magneto-optic Faraday effect. Since the light is coupled in substantially parallel to the layer according to the invention, the optical path of the light is increased by several orders of magnitude compared to coupling in substantially perpendicular to the base surface as known from the prior art. Accordingly, the angle of polarization rotation increases so that a distinct increase in sensitivity is obtained.
According to claim 9, it is provided that the magneto-optic layer consists at least partly of iron garnets and is applied to a substrate consisting at least partly of gallium garnets, whereby oxygen is substituted at least partly by sulfur in the gallium garnets of the substrate. Iron garnets are formed by compounds based on iron garnet (RE3Fe5O12), whereby RE3 comprises three rare earth metal elements, and iron (Fe) and/or oxygen (O) can be substituted at least partly by one or more other elements in each case. Gallium garnets are compounds based on gallium garnet (RE3Ga5O12), whereby RE3 comprises three rare earth metal elements, and gallium (Ga) and/or oxygen (O) can be substituted at least partly by one or more other elements in each case.
The at least partial substitution of oxygen in the substrate by sulfur increases the lattice constant of the substrate, which in turn permits a substitution of rare earth metals, e.g. yttrium, in the magneto-optic layer by sensitivity-increasing elements, in particular bismuth, with a larger atomic and/or ionic radius. This permits the angle of polarization rotation, which is proportional to a material constant influenced by the composition of the magneto-optic material in the magneto-optic Faraday effect, to be distinctly increased.
Altogether, the apparatuses according to claim 1 and 9 permit more precise and reliable examination of even very small magnetic fields, in particular on bank notes. Simultaneously, the high sensitivity of the inventive apparatuses permits a relatively large distance between bank note and magneto-optic layer, so that high transport speeds of the bank notes under examination can be obtained along with lower wear and a distinctly reduced risk of jams.
The inventive apparatuses furthermore permit high spatial resolutions to be obtained, so that they are especially suitable for examining the magnetic properties of printed images produced with magnetic ink, of magnetic layers or of magnetic, in particular interrupted or coded, security threads or security bands on or in bank notes or security documents.